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 | Dr Simon C BenjaminRoyal Society University Research Fellow |
[ Quicklinks: Research Summary Current Research Projects Recent Publications D.Phil. Projects Available ]
Summary of Research Interests
Physics of computation. Design and realization of architectures for new forms of information processing, especially quantum computing. Theoretical work relating to the design, growth and characterization of solid state nanostructures for computation, with particular current emphasis on (a) quantum dots systems, both self assembled and lithographically defined, and (b) fullerene systems (nanotubes, endohedral C60, etc.) Secondary interest in other areas of quantum information theory, such as quantum game theory.
Current Research Projects
Architectures for Computation in the Quantum Regime.
Dr. S.C. Benjamin
This project is concerned with the question, what is the best (most natural) architectural scheme for processing information at the quantum level? The issue is examined in two distinct contexts: processing classical information (bits) with quantum-scale structures, and processing of true quantum information (qubits). Although there are many topics of interest within this area, research into the ""global control"" concept is currently of primary interest. Using this approach bit/qubits can be effectively manipulated even if they are too close together to address individually. However, fundamental issues must be addressed before this novel paradigm can be considered mature: to determine the minimum space/time costs implied by adopting such a scheme, to quantitatively analyse fault tolerance (esp. for QIP), and to understand relative merits of one-, two- and three-dimensional arrays. (Funded by The Royal Society)
Cluster-state quantum computing in quantum optics
Dr. S.C. Benjamin, Dr. D. Browne, Dr. B.W. Lovett, J. Fitzsimons, Professor G.A.D. Briggs
Cluster-state quantum computing is a recent alternative to circuit-based quantum computing. It seems particularly suited for physical implementations that offer only probabilistic gates, such as linear optical quantum computing. Recently, the cluster-state formalism was adopted to show how quantum computing can be implemented in isolated quantum systems with optical transitions. This project encompasses the theoretical study of creating cluster states efficiently, and the effects of physical noise on its quantum computing capability.
Graph States and Projective Measurement Based Quantum computation (theory)
Dr. S.C. Benjamin, Dr. D. Browne, Dr. P. Kok, E. Campbell, P. Danvirutai, J. Fitzsimons
Graph state (or Cluster-state) quantum computing is a recent alternative to circuit-based quantum computing. It seems particularly suited for physical implementations that offer only probabilistic gates, such as linear optical quantum computing. Recently, the cluster-state formalism was adopted to show how quantum computing can be implemented in isolated quantum systems with optical transitions. This project encompasses the theoretical study of creating cluster states efficiently, and the effects of physical noise on its quantum computing capability. (In collaboration with Imperial College, London) (Supported in part by the QIPIRC (see www.QIPIRC.org), and by the Royal Society.)
Measurement based quantum entanglement of solid state spin qubits
F. Grazioso, P. Dolan, Dr. J.M. Smith, Dr. B.W. Lovett, and Dr. S.C. Benjamin
Recent advances in the theory of quantum information processing have shown that high fidelity quantum entanglement can be generated between remote systems simply by performing measurements on them in a certain way. From a materials perspective this is a huge advantage over traditional QIP schemes, which require controlled interaction between two or more systems - usually on nanometre length scales - to generate the necessary entanglement. Here we are developing the capability to perform a basic quantum measurement from which a measurement-based entanglement apparatus will be built. The ‘qubit’ that we are measuring is the spin state of a negatively charged Nitrogen Vacancy (NV-) defect in diamond; a system that has been demonstrated to have excellent coherence properties and on which some exquisite single qubit manipulation experiments have already been performed. Our focus is currently on characterising the optical properties of single NV- defects in ultra-pure diamond produced by Element Six Ltd, and on controlling the coupling of light between the defect and the measurement apparatus. The project involves collaboration with Element Six Ltd, the University of Melbourne, the University of Bristol, and the Institute of Photonics at the University of Strathclyde. Funding is provided by the EPSRC and MoD through the QIP IRC, and by Hewlett Packard Laboratories in Bristol.
Supramolecular structures for nanoelectronics and quantum computing
A. Shaw, Dr. M.R. Castell, Professor G.A.D. Briggs, Dr. A. Ardavan*, Dr. S.C. Benjamin, Dr. K. Porfyrakis
Conventional lithographic techniques for surface patterning have powered technological progress for decades and can now reach dimensions down to 20-30 nm. We shall pursue a fundamentally different approach to templating based on a 'bottom-up' nanotechnology in which nanoscale building blocks spontaneously adopt an ordered configuration through a self-assembly process. We shall use these templates to create structures suitable for nanoelectronics and quantum computing. The primary approach will be deposition of a passive molecular 'scaffolding' followed by subsequent deposition of a molecular species that forms an ordered distribution within that scaffolding. The species employed will be an endohedral fullerene with the property that the encapsulated atom can store information in the state of its nuclear and/or electron spin. In this way we shall create ordered arrays of quantum bits (qubits). Experiments will be designed to characterise the qubit-qubit intereactions, and the results will be used to guide further generations of nanoarray synthesis, with the ultimate goal of creating structures suitable for information processing. (*Department of Physics)
Molecular Architectures Templated by DNA
Dr. A. Ardavan*, Dr. S.C. Benjamin, Professor G.A.D. Briggs, Dr. R. Goodman*, Dr.A.N. Khlobystov**, Dr. J. Malo*, Dr. A. Turberfield*
We aim to establish a technology capable of using self-assembling DNA scaffolding to create functional architectures of molecular-scale components with the potential to perform computation. In our preferred system quantum information will be embodied in electron spins on atoms doped within fullerene cages, which are attached to a DNA lattice by covalent bonding. (*Clarendon Laboratory, Department of Physics; **University of Nottingham)
Nanomaterials and quantum computing
Dr. B.W. Lovett, Dr. S.C. Benjamin and E.M. Gauger
We are looking at how certain nanomaterials (such as quantum dots or crystal defects) can be used to implement quantum gate operations. We have developed methods for coherent quantum control of systems with a range of Hamiltonians. We are also interested in modelling decoherence, which is caused by the interaction of a system with its environment, and employs the theory of open quantum systems. We look at both Markovian and non-Markovian models of such open systems. We aim to provide experimental tests of the different theories working closely with the Low Dimensional Structures and Devices group at the University of Sheffield (http://ldsd.group.shef.ac.uk/).
Measurement Based Quantum Computing
Dr. B.W. Lovett and Dr. S.C. Benjamin
One can regard quantum entanglement as the fundamental resource needed in order to execute quantum algorithms. Certain kinds of entangled states exist which are universal resources, in the sense that any quantum algorithm can be performed simply by performing a prescribed series of quantum measurements. Moreover, even the entangled state itself can by created by making measurements. These insights have led to many new possible implementations of quantum computers, for example: one that uses only photons, one exploiting crossed atomic beams and others based on optical measurements on colour centres in diamond.
Specific topics are: first principles physics of measurement, implementation of error correction or avoidance and entanglement creation by measurement.
Coherent Control of Spin Systems
Dr. B.W. Lovett, Dr. S.C. Benjamin, E.M. Gauger
We are studying the quantum properties of nuclear and electron spins, primarily in molecular systems. Our aim is to provide theory that will allow for the control small numbers of spins, such that the quantum coherence is preserved for as long as possible. We collaborate with the Quantum Spin Dynamics experimental group (http://qsd.physics.ox.ac.uk/), and together we demonstrated that the quantum state of an electron spin can be transferred coherently to a nuclear spin, thus increasing the coherence time. We are now working on optical methods for further improving coherence, and for coupling several spins together.
9 public active projects
Research Publications
Benjamin, S.C., Ardavan, A., Andrew, G., Briggs, D., Britz, D.A., Gunlycke, D., Jefferson, J., Jones, M.A.G., Leigh, D.F., Lovett, B.W., Khlobystov, A.N., Lyon, S.A., Morton, J.J.L., Porfyrakis, K., Sambrook, M.R. and Tyryshkin, A.M. (2006). 'Towards a fullerene-based quantum computer' Journal of Physics-Condensed Matter 18(21) S867-S883.
Benjamin, S.C., Browne, D.E., Fitzsimons, J. and Morton, J.J.L. (2006). 'Brokered graph-state quantum computation' New Journal of Physics 8.
Morton, J.J.L., Tyryshkin, A.M., Ardavan, A., Benjamin, S.C., Porfyrakis, K., Lyon, S.A. and Briggs, G.A.D. (2006). 'Bang-bang control of fullerene qubits using ultrafast phase gates' Nature Physics 2(1) 40-43.
Tyryshkin, A.M., Morton, J.J.L., Benjamin, S.C., Ardavan, A., Briggs, G.A.D., Ager, J.W. and Lyon, S.A. (2006). 'Coherence of spin qubits in silicon' Journal of Physics-Condensed Matter 18(21) S783-S794.
Yung, M.H., Benjamin, S.C. and Bose, S. (2006). 'Processor core model for quantum computing' Physical Review Letters 96(22).
Benjamin, S.C. (2005). 'Comment on "Efficient high-fidelity quantum computation using matter qubits and linear optics"' Physical Review A 72(5).
Benjamin, S.C., Eisert, J. and Stace, T.M.: 'Optical generation of matter qubit graph states' New Journal Of Physics 7 (2005)
Benjamin, S.C. and Bose, S.: 'Quantum computing in arrays coupled by "always-on" interactions' Physical Review A 70 (3) (2004)
Benjamin, S.C., Lovett, B.W. and Reina, J.H.: 'Optical quantum computation with perpetually coupled spins' Physical Review A 70 (6) (2004)
Benjamin, S.C.: 'Multi-qubit gates in arrays coupled by 'always-on' interactions.' New Journal Of Physics 6 (2004) art. no.-61.
Ardavan A., Austwick M., Benjamin S.C., Briggs G.A.D., Dennis T.J.S., Ferguson A., Hasko D.G., Kanai M., Khlobystov A.N., Lovett B.W., Morley G.W., Oliver R.A., Pettifor D.G., Porfyrakis K., Reina J.H., Rice J.H., Smith J.D., Taylor R.A., Williams D.A., Adelmann C., Mariette H. and Hamers R.J.: 'Nanoscale solid-state quantum computing' Philosophical Transactions of the Royal Society of London Series A - Mathematical Physical and Engineering Sciences 361, 1473-1485 (2003).
Benjamin S.C. and Bose S.: 'Quantum computing with an always-on Heisenberg interaction' Physical Review Letters 90, art. no.-247901 (2003).
Projects Available
Quantum Information Processing
B W Lovett / S C Benjamin
The Quantum and Nanotechnologies Group (www.qunat.org) anticipates that they will be able to offer one or more doctoral studentships in the area of quantum information processing. The group has broad interests, ranging from detailed modelling of semiconductor structures through more abstract ideas related to designs for quantum computer architectures and extending to fundamental questions about the nature of quantum information and measurement. At the time of writing the following are active projects:
i) Measurement based quantum computing. One can regard quantum entanglement as the fundamental resource needed in order to execute quantum algorithms. Certain kinds of entangled states exist which are universal resources, in the sense that _any_ quantum algorithm can be performed simply by performing a prescribed series of quantum measurements. Moreover, even the entangled state itself can by created by making measurements. These insights have led to many new possible implementations of quantum computers, for example: one that uses only photons, one exploiting crossed atomic beams and others based on optical measurements on colour centres in diamond.
Specific topics are: first principles physics of measurement, implementation of error correction or avoidance and entanglement creation by measurement.
ii) Nanomaterials and quantum computing. We are looking at how certain nanomaterials (such as quantum dots, molecules or crystal defects) can be used to implement quantum gate operations. We have developed methods for coherent quantum control of systems with a range of Hamiltonians. We are also interested in modelling decoherence, which is caused by the interaction of a system with its environment, and employs the theory of open quantum systems.
iii) Spin chains. One of the most important questions in quantum information processing is how we might transmit information from one computer to another. We have been looking at at this might be done using one (or higher) dimensional arrays of interacting spins (or similar quantum two level systems). An important theme is to achieve is much as possible with minimal external control --- in other words, to exploit the 'natural' dynamics of the spin system as completely as possible.
Another potential application of a spin chain is as a globally controlled quantum memory element. We are interested in developing the theory of molecular quantum memories, for both interacting and independent molecular systems.
There are several collaborators on these projects, including Dr Tom Stace (University of Queensland), Prof Sougato Bose (University College London), and Prof Leong Chuan Kwek (National University of Singapore). Currently no specific funding is in place; however, a number of funding routes exist and we would be happy to advise strong students about how to explore these.
Also see homepages:Simon Benjamin Brendon Lovett
Also see a full listing of New projects available within the Department of Materials.
